Thermomechanical treatment process for heat treatable aluminium alloys

ABSTRACT

Heat treatable aluminum alloys of increased mechanical strength are produced by submitting the alloy to a solution and quenching treatment followed by a combination of plastic deformations and precipitation hardening, including a first aging treatment at a temperature not higher than 200*C, a plastic deformation and then a further aging at a temperature not lower than the first, the combination for deformations and hardenings being a cycle repeatable on the alloy.

United States Patent Di Russo et al.

[ Dec. 19, 1972 THERMOMECHANICAL TREATMENT PROCESS FOR HEAT TREATABLEALUMINIUM ALLOYS Inventors: Ettore Di Russo; Mario Conserva,

both of Novara, Italy Assignee: Societa per LEsercizio dell lstitutoSperimentale dei Metalli Leggeri S.p.A, Milan, Italy Filed: April 14,1970 Appl. No.: 28,514

Foreign Application Priority Data Feb. 10, 1970 Italy ..20,399 A/70 US.Cl ..l48/12.7

..C2 2f l/04 Field of Search ..l48/l27, 159

References Cited UNITED STATES PATENTS 6/1937 Nock v.148/159 3,454,4357/l969 Jacobs ..l48/l2.7

2,249,349 7/l94l Deutsch l48/l 2.7 3,198,676 8/1965 Sprowls et al...l48/l59 Primary ExaminerW. W. Stallard Alt0rneyStevens, Davis, Miller& Mosher 57] ABSTRACT 3 Claims, No Drawings THERMOMECHANICAL TREATMENTPROCESS FOR HEAT TREATABLE ALUMINIUM ALLOYS The present inventionconcerns a thermomechanical process particularly suitable to achieve aremarkable increase in the mechanical strength of semifinished heattreatable aluminum alloys without negatively affecting the secondaryproperties thereof.

This invention particularly comprises specific thermomechanicaltreatments specially suitable for aluminum alloys of the systemAl-Zn-MG; Al-Zn-Mg-Cu; Al-Cu; Al-Cu-Mg; Al-Cu-Mg-Si; Al-Si-Mg; saidalloys being particularly useful for the aeronautical and motorindustries as well as, more extensively for constructional partsrequiring high mechanical strength. The improvements in the alloyproperties can be attained, according to the present invention, throughthermomechanical cycles based on suitable combinations of plasticdeformations and hardening by precipitation; by these treatments it isin fact possible to induce in the materials a condition ofsuper-hardening related to the combined effect of the hardening phasesand of a stable structure of a dislocation network induced by thedeformation.

The thermomechanical cycle according to the present inventionsubstantially differentiates from the combined cycles deformation andaging suggested for heat treatable Al alloys by prior art disclosuresboth in consequence of the different sequence of the various phasesmaking up a thermomechanical cycle and also owing to the particulartemperatures used for some intermediate phases; the sequence of phasesaccording to this invention allows in fact one to obtain, particularlyfor some kinds of alloys, better combinations of static strength andsecondary characteristics by virtue ofthe fact that a realsuper-hardening condition, unobtainable through the knownthermomechanical cycles, is induced in said alloys.

Particularly, the known combined treatments so far used for Al alloys ofthe systems Al-Cu-MG, Al-Cu, Al- Si-Mg and Al-Zn-MgCu do not lead tosuch results as are obtainable through the herein proposed sequence ofcycles, since said known treatments do not induce any superhardeningcondition characterized by high structural stability.

An object of the present invention is to provide a particularthermomechanical process suitable to induce in said heat treatable Alalloys a super-hardening such as to grant them remarkable improvementsin their static strength characteristics without negatively influencingthe other properties of said alloys.

Another object of the present invention is to provide a process which,by establishing such a super-hardening, succeeds in improving somesecondary characteristics of said alloys without significantly alteringother secondary characteristics while always producing a staticmechanical strength not below but often above that obtainable on thesame alloys through traditional treatments.

These objects are advantageously reached through a thermomechanicalprocess suited to induce in the heat treatable Al alloys asuper-hardening characterized by its high structural stability, saidprocess according to this invention comprising submitting said aluminumalloys, preferably in the form of bars, sheets plates-and like preformedshapes to a solution and quenching (T) treatment in water or blast air,followed by a combination of plastic deformations and precipitationhardenings including: a first aging treatment (A at temperature below200C, for a time not longer than 50 hours, a plastic deformation (H),preferably a thickness or section reduction and then a further aging (Aat a temperature and for a time preferably not smaller than the onesused for said first aging treatment, said combination of deformationsand hardenings (A HA making up a treatment cycle repeatable attemperatures and for times preferably different from the mentioned oneson the same alloy sample, for further improving its mechanicalproperties.

More particularly said thermomechanical process comprises for the alloyshaving Cu as the main alloying element, a plastic deformation (H) notbelow 5 percent, after said quenching treatment (T). The general schemeof the thermomechanical treatment which is within the scope of thisinvention and applicable to heat treatable Al alloys, is of the type: TAHA TA l-lA l-lA or type THA1HA2;THA1HA HA wherein T stands for asolution (performed at various temperatures and for different heatingtimes) and quenching carried out in various media and at various coolingrates,

A, represents an aging treatment developed (through one or more stages,at various temperatures and for different heating times) till obtainingin the material a distribution of precipitated or preprecipitated zones,coherent and/or semicoherent, with a stability degree such as to obtainthe effectiveness of the selected thermomechanical cycle in itsfollowing stages,

H stands for a plastic deformation brought about, with cold or warmprocessing, in a way and at values such as to let the material acquirehigh density and homogeneous distribution of dislocations network whileretaining also, after its deformation, the possibility of a followinghardening due to agmg.

A A etc. represent intermediate or final steps of aging carried out atselected temperature and through suitable times, in such a way as toinduce in the material a composed structure of dislocations and of afinal precipitation hardening as favorable as possible for the finalcharacteristics wished for the material.

It has in practice been found that any semifinished product of heattreatable Al alloy of commercial type can be advantageously treated withthe process according to this invention; it has been also found that thebest results are obtained when the concerned process is applied tosemifinished materials coming from ingots or plates submitted toparticularly strong cycles of homogenization, as well as when thisprocess is applied to semifinished material coming from alloys preparedwith high purity Al and from alloys containing particular additionalelements, mainly when said semifinished materials are submitted tostrong homogenization cycles.

The thermomechanical process according to this invention is particularlysuitable for treating Al alloys of the type: Al-Zn-Mg: Al-Zn-Mg-Cu;Al-Cu, Al-Cu-Mg; Al-Cu-Mg-Si; Al-Si-Mg.

To better evidence the results obtainable through this process, someexamples of treatment of Al alloys are given hereinafter forillustration only and, are not to be construed as limiting of theinvention in scope.

In addition to the treatment of this invention, known treatments arealso reported in said examples, and included tables, to better evidencethe remarkable improvements obtained in the final characteristics of thematerials.

it is also to be noted that in said examples the known treatments arecarried out using the best known conditions for obtaining the bestresults in properties, so that these results can be compared with theresults of the cycles according to this invention.

In said examples: Rm is the mechanical tensile strength, in kg/mm Rp(0.2) is the permanent deformation limit load, in

kg/mm A, stands for the ultimate elongation, in percent. 0

EXAMPLE 1 Five rolled sheets (thickness 5 mm) of commercial alloy withcomposition Al--5.l% Zn 1.1% Mg 0.14% Cr 0.21%Mn 0.12% Zr 0.12% Fe 0.10%Si were submitted to quenching (after p're-heating at 460C for 1 hour)in water at room temperature, every sheet treated in this way beingafterwards submitted to a different thermal or thermomechanical cycle.Said cycles, each for every sheet, were as follows:

TA (known cycle isothermal (one step) aging) TA,A (known cycle-di-isothermal (two step) aging) THA,A (known cycle TA,A,H (known cycleTA HA, (cycle according to this invention) wherein H represents a 20percent reduction in thickness obtained through rolling at roomtemperature, parallel to the original rolling direction Mechanicaltensile tests were carried out along the rolling direction correspondingto the hardness peaks of every treatment cycle and the ascertainedresults are shown in the following table 1.

TA,HA, 100 100 120 44.3 42.2 12.4

This table clearly evidences that cycle TA,HA,, according to the presentinvention, gives the best results and particularly the highestcombination of strength and ductility.

EXAMPLE 2 Two portions vof commercial plate of 35 mm thickness withcomposition Al 4.35% Zn 2.06%

, Mg-- 0.15% Cr--0.22% Mn-0.10% Zr--0.17% Fe 0.15% Si were submitted toa solution treatment at 460C for two hours under steady conditions; onesample was then drastically quenched in cold water (T), while the otherwas quenched in a stream of air (Ta.v.

Three samples of the first portion previously treated in cold water,were submitted to thermal and thermomechanical (one for every specimen)treatments according to the followingv cycles: TA A,- TA HA, TA HA,while three further samples of the second portion, previously treated inan air stream, were submitted to treatments (one for every. sample) ofthe type: Ta.v.A,A-, Ta.v.A HA, Ta.v.A HA,.

H represents a 20 percent plastic deformation (thickness reduction)brought about on each of the six samples by rolling them, at roomtemperature, parallel to the direction of the original working.

Specimens were taken from the long transverse direction of said samples,to ascertain mechanical characteristics (Rm, Rp and A and stresscorrosion resistance (by continuous dip in a 3% NaCl solution bufferedat pH 5.2, being the tensile stress applied to the specimen equal topercent of Rp. (0.2). The following Table 2 shows the results which wereobtained:

The table clearly evidences that both thermomechanical cycles TA HA andTa.v.A HA, being within the present invention, lead to remarkableimprovements.

In case of water cooling the stress corrosion resistance is improvedwithout involving any decrease in the mechanical properties of thematerial. In case of air stream/cooling a stress corrosion resistanceequal to the one attained through known treatments is maintained butthemechanical properties are improved.

EXAMPLE 3 Two extruded rod portions (diameter 15.2 mm) of commerciallyproduced alloy consisting of Al 5.07% Zn 0.96% Mg 0.14% Cr 0.22% Mn0.12% Zr 0.13% Fe 0.10% Si were submitted to quenching (after preheatingfor 2 hours at 460), in water at room temperature and subsequentlyunderwent two different thermal and thermomechanical cycles (a cycledifferent for every rod portion) of the type TA,A, and TA,HA,, wherein Hstands for a 20 percent deformation given by drawing at 25C abt. Somespecimens were taken from the two portions treated as above, andsubmitted to mechanical tensile strength tests and rotating cantileverfatigue tests (un- EXAMPLES Two sheets (thickness 2 mm) were rolled froma chill cast ingot (of 110 mm diameter) (cooled in water notchedspecimens): the specific results obtained 5 after casting) ofcomposition Al 8.02% Zn 2.52%

thereby are shown in the following Table 3.

TABLE3Y Mg 1.58% Cu 0.20% Mn 0.18% Zr 0.002%

By the shown data it is seen that the cycle according to this inventionhighly increases the static strength characteristics, in comparison witha traditional cycle TA,A,. It is also to be noted that the fatiguecharacteristics displayed by material treated according to TA,HA aresubstantially analogous to those noticed on material treated accordingto TA,A,.

EXAMPLE 4 Six rolled strips (thickness 2 mm) of commercially produced Alalloy having composition Al 5.5% Zn 2.5% Mg 1.7% Cu 0.20% Mn 0.13% Cr0.28% Fe 0.20% Si 0.08 Tiwere submitted to quenching in water at roomtemperature (after preheating for one hour at 460C), followed by variousthermal and thermomechanical treatment cycles (a different cycle forevery strip) of the type TA, TA,A, TAJ-lA TA,1-1A TA,H TA,1-1,A,H,Awherein H represents a deformation (given by rolling at 25C and 150Cabt.) produced by working parallel to the original rolling direction,said deformation amounting to percent, percent, 30 percent.

Tensile tests (being the specimen axis parallel to the original rollingdirection) were carried out correspond ing to the hardness peaks ofevery treatment cycle and the results obtained appear in the followingTable 4:

The values used for the last cycle in Table 4, (TA,H, A ll A were: A,105C for 6 hours, H, 20%, A 105C for 6 hours, H 10%, A, 105C for 15hours.

Fe 0.0023% Si, after intensive homogenization at high temperature andnormal hot rolling cycle. Said sheets were submitted to thermal andthermomechanical treatments (a treatment different for every sheet) ofthe type TA and TA,1-1A where T =quenching carried out (after 1 hourheating at 470C) by cooling in water at room temperature, H being 10%percent brought about by rolling at C, parallel to the original rollingdirection.

The results which were obtained appear in Table 5:

TA,HA,

This table shows that the cycle according to this invention produces abetter combination of strength and ductility in comparison with theresults reached through conventional cycle TA.

EXAMPLE 6 Some samples (diameter 14 mm), were extruded from a l 10 mmdiameter chill cast ingot of composition A1-- 6.23% Cu 0.39% Mn 0.20% Zr0.19% Cd 0.06% Ti 0.16% Fe 0,07% Si, after a traditional cycle ofhomogenization. Treatments of the type TA, and TA,1-1A, were carried outon said extruded samples, where T quenching realized (after two hours ofpreheating) by cooling from 510 to 20C in water.

The results which were obtained appear in following Table No. 6, wherein11 represents a 20% reduction in section given by cold drawing.

This table evidences a remarkable increase in the yield strength as wellas a very slight decrease in the elongation attained with the cycle ofthis invention, in comparison with the results reached throughconventional cycle TA,; it is therefore quite evident that thecombination of properties obtained through the cycle of this inventionis superior.

' EXAMPLE 7 Rolled sheets (thickness 2 mm) of commercially producedalloy Al 4.5% Cu 1.5% Mg 0.56% Mn 0.33% Fe 0.14% Si, were submitted,after quenching in room temperature water (previous preheating for onehour at 495C) to various thermal and thermomechanical cycles of the typeTA THA TA,H TA HA TH A H A wherein H represents a deformation obtainedby rolling at 25C, parallel to the original rolling direction. Thetensile tests (performed with specimen axis parallel to the rollingdirection) are shown in following Table No. 7.

TABLE NO. 7

Rm Rp (0.2) A, Treatment Kg/mm Kg/mm Type A,=190C for 48.6 40.2 14.5

A, hours Type A,=190C for 8 53.7 52.9 8.1 THA hours H 6% (obtained bycold rolling) Type H=40% (obtained by 58.2 56.2 6.6 THA, cold rolling A,190C for 2 hours Type A 190C for 10 58.9 56.7 3.5 TA H hours H 40%(obtained by cold rolling) Type A 140C for 8 t 60.9 54.6 11.4 TA HA,hours H 40% (obtained by cold rolling) A,= 140C for 8 hours Type A 140Cfor 6 56.1 51.8 12.6 TA HA, hours H 20% (obtained by cold rolling) A,=180C for 4 hours Type H 6% (obtained 58.4 51.9 13.4 TH,A, by coldrolling) H,A, A 140C for 12 hours H,= 20% (obtained by cold rolling) A,140C for 20 hours EXAMPLE N0. 8

Rolled sheets (thickness 2 mm) of commercially produced alloy Al 0.98%Si 0.73% Mg 0.62Mn 0.27% Fe,,were submitted (previous quenching carriedout after steady preheating of 1 hour at 530C and cooling in water atroom temperature) to various thermal and thermomechanical cycles of thetype TA TA,H TA HA, wherein H represents a deformation given by rolling,at 25C abt., parallel to the original direction of working.

The tensile tests (performed with the specimen axis parallel to therolling direction) are shown in following Table No. 8.

TABLE 8 Rm Rp(0.2) s Treatment Kg/mm' Kg/mm' Type A, 175C for 8 37.134.5 .14.5

A, hours Type .A. C for 10 37.0 34.0 6.0 TA,H hours The above givenexamples clearly demonstrate that the combined thermomechanicaltreatment (TA,HA being within the present invention surely and alwaysleads to a remarkable increase in the mechanical strength withoutnegatively influencing the secondary properties (in some cases on thecontrary improving some of them) such as ultimate elongation, stresscorrosion tensile strength, fatigue strength, creep strength etc.

It is obvious that modifications and alternatives with equivalentcharacter can be made while remaining within the spirit and scope of theinvention.

What is claimed is:

l. Athermomechanical treatment process for heat treatable aluminumalloys selected from the group consisting of Al-Zn-Mg and Al-Zn-Mg-Cusystems comprising submitting said alloys in preformed shape to asolution and quenching treatment (T) followed by a combination ofplastic deformation and precipitation hardening including:

a first aging treatment (A,) at a temperature between 60 and 200C fortimes ranging from 2 to 50 hours,

a plastic deformation (H) of between 10 and 30 percent obtained byreducing the thickness of the wrought product and a further agingtreatment (A,) at a temperature

2. The process of claim 12 wherein said second aging treatment (A2) iscarried out at a temperature between 60* and 230*C for times rangingfrom 2 to 120 hours.
 3. The process of claim 12 wherein said combinationof deformation and hardenings (A1HA2) is a cycle repeatable on the samealloy sample at temperatures and for times in the same range as thosespecified.